The present invention relates to cathode-ray tubes, and particularly to color cathode-ray tubes of the type useful in color display systems. The invention is especially applicable to self-converging tube-yoke combinations with cathode-ray tubes of the type having plural-beam inline guns disposed in a horizontal plane.
An inline electron gun is one designed to generate a trio of electron beams in a common plane and to direct the beams along convergent beam paths to a small area spot on a phosphor screen. A self-converging yoke is one designed with specific field nonuniformities which maintain the beams converged throughout the raster scan without the need for convergence means other than the yoke itself.
In one type of inline electron gun, such as that shown in U.S. Pat. No. 3,772,554, issued to R. H. Hughes on Nov. 13, 1973, a main electron lens for focusing the electron beam is formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having the bottoms of the members facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams. In such electron guns, static convergence of the other beams with respect to the center beam is usually attained by offsetting the outer apertures in the second focusing electrode with respect to the outer apertures in the first focusing electrode.
It has been noted that the horizontal beam landing locations of the outer electron beams, in color picture tubes having the above-described electron gun, change with changes in the focus voltage applied to the electron gun. It therefore is desirable to improve such inline electron guns to eliminate, or at least reduce, this horizontal convergence sensitivity to focus voltage changes.
Additionally, there has been a general trend toward inline color picture tubes with greater deflection angles (angles in excess of 90.degree.) in order to provide shorter tubes. In such tubes, it has been found that the electron beams become excessively distorted as they are scanned toward the outer portions of the screen. Such distortions are commonly referred to as flare and appear on the screen of the tube as an undesirable low intensity tail or smear extending from a desirable intense core or spot. Such flare distortions are due, at least in part, to the effects of the fringe portions of the deflection field of the yoke on the beam as it passes through the electron gun, and to the nonuniformities in the yoke deflection field itself.
When the yoke's fringe field extends into the region of the electron gun, as is usually the case, the beams may be deflection slightly off axis and into a more aberrated portion of an electron lens of the gun. The result is frequently a flare distortion of the electron beam spot which extends from the spot toward the center of the screen. This condition is particularly troublesome in self-converging yokes having a toroidal vertical deflection coil, because of the relatively strong fringing of toroidal type coils.
Self-converging yokes are designed to have a nonuniform field in order to increasingly diverge the beams as the horizontal deflection angle increases. This nonuniformity also causes vertical convergence of the electrons within each individual beam. Thus, the beam spots are overconverged at points horizontally displaced from the cente of the screen, causing a vertically extending flare both above and below the core of the beam spot.
The vertical flare due to both the effects of the yoke's fringe field in the region of the gun and to the nonuniform character of the yoke field itself is an undesirable condition which contributes to poor resolution of a displayed image on the edge and corners of the screen.
U.S. Pat. No. 4,513,222, issued to Chen on Apr. 23, 1985, and copending U.S. patent application Ser. No. 492,044, filed on May 6, 1983, by Chen and now U.S. Pat. No. 4,523,123 and assigned to the assignee of the present invention, each disclose screen grid structures for simultaneously reducing both the horizontal sensitivity of the outer beams of the inline electron gun to focus voltage changes and the vertical flare distortion of the electron beam spot. The disclosed structures utilize a plurality of rectangular slots aligned with the screen grid apertures and formed in the surface of the screen grid facing the control grid to create an astigmatic field that produces underconvergence of the electron beam in the vertical plane only, to compensate for the vertical flare distortion. Such a slot structure is described in U.S. Pat. No. 4,234,814, issued to Chen et al. on Nov. 18, 1980.
The screen grid structure disclosed in U.S. Pat. No. 4,513,222 utilizes a pair of reconvergence slots formed on the first focusing electrode side of the screen grid to compensate for the offset refraction within the main lens of the electron gun due to focus voltage changes. The reconvergence slots are formed closely to and inwardly from the outer apertures in the screen grid and cause a refraction of the electrostatic beam path between the screen grid and the first focusing electrode.
The screen grid structure disclosed in the copending patent application Ser. No. 492,044 utilizes a pair of circular depressions formed asymmetrically about the outer apertures on the first focusing electrode side of the screen grid to reduce the horizontal convergence sensitivity, within the main lens of the electron gun, due to focus voltage changes. The circular depressions are precisely displaced toward the center aperture of the screen grid.
The aforedescribed structures have a plurality of rectangular slots aligned with the screen grid apertures on one side of the screen grid to compensate for vertical flare, and either reconvergence slots, formed inwardly of the outer apertures, or circular depressions, formed asymmetrically about the outer apertures, on the opposite side of the screen grid, to reduce the horizontal convergence sensitivity of the outer beams due to focus voltage changes. Such structures require precise placement of the rectangular slots as well as the reconvergence slots and circular depressions relative to the apertures and are, therefore, expensive to manufacture. Thus, a screen grid, which corrects both vertical flare and horizontal convergence sensitivity to focus voltage change and is easily and inexpensively manufactured, is desirable.
Copending U.S. patent application Ser. No. 492,437, filed on May 6, 1983, by van Hekken et al. and now U.S. Pat. No. 4,520,292, discloses a screen grid having a refraction lens comprising a recessed portion formed in the surface facing the main electron lens. A peripheral rim, which makes an angle of about 63.degree. with the surface of the electrode, surrounds the recessed portion through which the screen grid apertures are formed. The refraction lens provides a correction for the horizontal convergence sensitivity to focus voltage change. In order to correct for vertical flare in tubes have a deflection angle in excess of 90.degree., a slot is superposed on each of the apertures of the control grid on the side facing the screen grid. The slots are symmetrically disposed about the control grid apertures and extend in a direction perpendicular to the plane of the apertures of the inline gun. Such a control grid structure is disclosed in copending U.S. patent application Ser. No. 485,860 by Bechis et al. on Apr. 18, 1983 and now U.S. Pat. No. 4,558,253. The aforementioned structure requires precise forming of the slots in the control grid and of the recessed portion in the screen grid in order to reduce vertical flare and horizontal sensitivity to focus voltage change, respectively. The precise forming of two grids, the control grid and the screen grid, of the electron gun is even more expensive than the previously described screen grid structures providing both flare reduction and correction of horizontal convergence sensitivity to changes in focus voltage.